The present invention relates generally to machine tools, and more particularly to a machine tool provided with a plurality of machining regions wherein a plurality of palettes each detachably mounting a workpiece are movable into and away from its associated machining region.
In undergoing machining operations with respect to the plurality of palettes on a palette basis, a wanted sub-program is conventionally called to merge it with a relevant program, thereby forming a machining program suitable for the intended machining operation. Alternatively, with an external program selection device having program selection switches, an intended machining program is selected by the manipulation of the selection switches.
The former is, however, disadvantageous in that the palette to be machined is selected by the machining program and that the order of the palette selection is definitely determined and cannot be determined arbitrarily. While the latter is capable of arbitrarily determining the palette selection, another disadvantage is encountered in that it is necessary to select a machining program with respect to the selected palette by designating a program number assigned to the intended machining program through a sequencer. It is therefore intricate to provide a device for selecting a program number corresponding to the intended machining program. Moreover, when the machining program applied to the workpiece mounted on the palette is replaced with another, it takes considerably long time to operate the sequencer and select the relevant program.
When the machining operation is implemented in accordance with the relevant program, it is necessary to check whether or not an end position to which a tool is finally moved is within an allowable movement range. If the end position of the tool is outside the allowable movement range, the machining operation must be stopped. Japanese Patent Publication (Examined) No. 59-47322 discloses a stroke over detection method complying with the above requirement.
However, the stroke over detection method disclosed in the Japanese Patent Publication is capable of detecting a stroke over status when the movement of the tool exceeds a single predetermined stroke. Therefore, in the machine tool in which a plurality of palettes are introduced into the corresponding machining regions and the machining operation is undergone on palette basis, an allowable maximum movement range applicable to all the machining regions needs to be determined, alternately an individual allowable movement range needs to set to each of the machining regions. The former is not satisfactory in the case where an error is contained in the machining program. Because, despite the fact that the tool should not be moved exceeding a predetermined position, the tool is allowed to move beyond that position insofar as the latter position is within the allowable movement range. The latter is also unsatisfactory in that setting of the individual allowable movement ranges is intricate and a setting error is liable to occur.
Further, a requirement exists in undergoing the machining operation in that the movement of the tool must be restricted so as not to enter into another machining region. Conventionally, the restriction of the movement of the tool is implemented by software. However, should there be an error in the software program or a runaway of a central processing unit, the intended control can not be accomplished and thus the reliability is lowered.
Moreover, when the workpieces are intended to be machined into the same shape or configuration, individual machining programs are needed corresponding to the plurality of palettes mounting the workpieces. However, a large quantity of memory is required to store the individual machining programs.